Lithium drifted germanium

The introduction of high-resolution, high-efficiency /-ray detectors composed of lithium-drifted germanium crystals has revolutionised /-measurement techniques. Thus, /-spectrometry allows the rapid measurement of relatively low-activity samples without complex analytical preparations. A technique described by Michel et al. [25] uses Ge(Li) /-ray detectors for the simultaneous measurements of 228radium and 226radium in natural waters. This method simplifies the analytical procedures and reduces the labour while improving the precision, accuracy, and detection limits. 

Chassery et al. [97] studied the 87Sr/86Br composition in marine sediments, observing excellent agreement between results obtained by ICP-MS and thermal ionisation mass spectrometry. Low level a-spectrometry with lithium drifted germanium detectors has been used to determine 90strontium in seawater [59]. 

As discussed above, the measurement of characteristic y rays is very similar to the methods used in EDXRF. Early studies used a scintillation counter, typically a crystal of sodium iodide containing a small amount of thallium (Tite 1972). y ray absorption by these counters produces visible light, which is converted into an electrical pulse using a photosensitive detector. More recently semiconductor detectors have been used, either a lithium drifted germanium crystal, or, more typically, a pure ( intrinsic )... 

Sayre, E. V. (1965). Refinement in methods of neutron activation analysis of ancient glass objects through the use of lithium drifted germanium diode counters. In Comptes Rendus Vile Congres International du Verre, Bruxelles, 28 Juin-3 Juillet 1965, Charleroi, Institut National du Verre. 

Radionuclidic analyses are performed with either a lithium-drifted germanium or intrinsic germanium detector. The assay for Sr-82 is based upon its 777 keV photon of 13.6% abundance. Strontium-85, which is often present in amounts comparable to that of Sr-82, is assayed by its 514 keV photopeak, which must be resolved from prominent 511 keV annihilation radiation by a curve stripping procedure (12). 

Lithium-Drifted Germanium Detectors. Two kinds of planar Ge(Li) diodes were fabricated in our Laboratory for the spectrometer (I). The chief consideration was maximum cross section with a resolution of less than 3 k.e.v. at 60Co FWHM (full width half maximum). The available single-crystal germanium material dictated the shape of the detector. The first detector was fabricated from a 15-cm. long Sylvania ingot with a trapezoidal cross section its finished dimensions were 6 cm. X 3 cm. X 1 cm. Our second detector was fabricated from circular Hoboken stock (NPC Metal and Chemical Co., Los Angeles) its finished dimensions were 2.8 cm. diameter X 1.2 cm. thick. 

Analysis. The analytical system used for gamma-ray measurements consisted of a lithium drifted germanium (GeLi) crystal detector, a 4096 multi-channel analyzer, a PDP 11 computer, and a cassette magnetic tape storage. The germanium detector crystal has a volume of 55 cm with FWHM resolution of 2.3 keV at 1.33 MeV. The computer was used to analyze the gamma ray spectra, to identify the radio isotopes, and to calculate the concentration (Table III). 

C19. Cooper, J. A., Evaluation of lithium-drifted germanium Compton-suppression spectrometers for non-destructive radiochemical analysis. J. Radioanal. Chem. 6, 177-184 (1970). 

Table III gives the conditions, data and reactions for the long irradiations of iridium, osmium and ruthenium. Samples and standards were irradiated in the University of London Reactor under a maximum thermal neutron flux of 1.4 x 10 n cm" s". The samples were counted using a lithium drifted germanium detector (Ortec Inc) linked to a computer based gamma ray spectrometer (Nuclear Data Inc. 6620 Multichannel Analyser). A general Neutron Activation Package written in FORTRAN IV was employed to run a peak search and calculate PGM concentrations in the plant samples. The irradiation of platinum is a special case and details are given in Table IV. Various nuclides emit y-rays of similar energy, and in INAA these become a serious interference in the determination of platinum in biological samples.

The availability of high flux thermal neutron irradiation facilities and high resolution intrinsic Ge and lithium drifted germanium (Ge(Li)) or silicon (Si(Li)) detectors has made neutron activation a very attractive tool for determining trace elemental composition of petroleum and petroleum products. This analytical technique is generally referred to as instrumental neutron activation analysis (INAA) to distinguish it from neutron activation followed by radiochemical separations. INAA can be used as a multi-elemental method with high sensitivity for many trace elements (Table 3.IV), and it has been applied to various petroleum materials in recent years (45-55). In some instances as many as 30 trace elements have been identified and measured in crude oils by this technique (56, 57). 

The Tm parent used for the 66-74-keV resonance does not populate the 75-89-keV level, and the shorter-lived Lu parent must be used [185]. This has a very complex decay, and a lithium-drifted germanium detector is required to detect the y-ray with sufficient discrimination. The Tm(a, 2n) Lu reaction provides a convenient route so that cubic... 

For truly multielement determinations, increased selectivity is required. It is offered by semiconductor detectors, e.g., by lithium-drifted germanium [Ge(Li)] or intrinsic germanium (high-purity) [HP-Ge] types with a resolution - full width at half maximum or FWHM... 

The neutron activation analysis was carried out at Livermore s 3-megawatt reactor. High resolution lithium-drifted germanium detectors were used to count the samples. Details of the irradiation and counting sequence can be found in an earlier paper (6). 

Accurate measurements of the concentrations of trace elements on atmospheric particulates are difiicult enough to make in polluted urban atmospheres but even more so in clean marine or polar atmospheres because of the minute quantities of material that can be collected in a reasonable time. For these measurements, one needs a sensitive analytical technique that is free from interference by other elements present. Recently, the use of lithium-drifted germanium [Ge(Li)] y-ray detectors in neutron activation analysis has greatly improved analytical sensitivities and accuracies for such studies (J). 

The lithium-drifted germanium detector, denoted Ge(Li), is similar in construction to the Si(Li) detector. However, a striking difference is found in the very high lithium mobility in the germanium detector at room temperature. A Ge(Li) detector will be destroyed if it is allowed to warm up from the normal 77 K operating temperature. The Si(Li) detector and preamplifier, on the other hand, usually can withstand a warm-up, providing the detector bias voltage is turned off. 

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